Primary current detectors are devices in which free carriers formed by the absorption of ionizing radiation pass into an external circuit without replenishment. See R. H. Bube, "Photoconductivity in Solids," John Wiley, New York (1960). Such devices include photoconductive elements used in electrostatic copy machines and other devices. Those devices can consist of a conductive substrate on which is mounted a blocking layer and a photoconductive layer of hydrogenated amorphous silicon. Before the photoconductive element is imagewise exposed to light, it must be charged, typically by means of a corona. Between the time of charging and the imagewise exposure to light, the photoconductive element remains in the dark. During this time, the element is subject to dark decay, which means that the charge placed on the element dissipates, either due to the injection of charge from the substrate or from the free surface of photoconductive layer, or by the thermal generation of charge in the material. To the extent that this occurs, the charge on the element is reduced. In addition, if materials from which the element is made are not uniform, the dark decay can occur in a nonuniform manner which can produce noise and a poor quality image. Efforts that have been made to reduce dark decay in these elements have concentrated on placing a barrier layer in between the conductive substrate and the photoconductive layer to prevent charges from passing therebetween. However, these efforts have not proved to be entirely satisfactory.